Flexible composite material and process for producing same

ABSTRACT

Composite material comprising a flexible sheath covering a roving of fibers impregnated to the core with a finely powdered thermoplastic, the melting point of which thermoplastic is equal to or greater than that of the sheathing material, together with process and apparatus for producing same by using an extrusion coating technique, the coating die being mounted on a leakproof powder fluidization tank via a blowing chamber thermally insulated from the tank.

BACKGROUND OF THE INVENTION

This invention relates to a novel composite flexible material, and moreparticularly, it relates to a composite material comprising a flexiblesheath covering a roving of continuous fibers of any type, be theymineral, organic or metallic, impregnated to the core with thermoplasticpowders, the melting point of which powders being greater than or equalto that of the flexible sheath, and to processes and apparatus forpreparing such composites.

A composite material has already been described in French parent patentapplication No. 83 10632. In this application, the melting point of thematerial used for the powder with which the fibers were impregnated was,however, restricted to a level considerably higher than that of thesheathing material's melting point.

The new composite material in accordance with the present invention isimportantly superior in its flexibility. The prior art composites, asshown in French patent application No. 81 21545, for example, althoughpliable, are rigid materials. They are in the form of strips, five toten mm in width and about one mm thick, and contain continuous parallelfibers like a reinforcement solidly embedded in a thermoplastic matrixwhich has been subjected to fusion and subsequent solidification bycooling.

Clearly, this type of material can only be rigid; experience also showsthat it breaks when bent.

An object of the present invention is to overcome this drawback whichproves a great handicap, in those applications in which the composite isto be placed around metal objects with a small bending radius (a radiusof 8-10 mm, for example), such as those encountered in production ofautomotive components (steering wheel frames, hydraulic cylinders,etc.).

THE INVENTION

In the process to which the invention refers, a roving of continuous,parallel--i.e., not twisted fibers is impregnated with particles ofthermoplastic powder, taking advantage of the momentary spreading out ofthese fibers when dipped in a fluidized bed of powder particles. Theseparticles of powder accumulate around each individual fiber in theroving due to electrostatic charges created using known techniques(electrostatic generator, friction brushes or rollers, and the like).

By gathering together these individual fibers loaded with a fine powderof thermoplastic material, a roving of parallel fibers impregnated tothe core with fine powder, and also covered on its periphery with alayer of the same fine powder, is obtained.

This roving loaded with thermoplastic fine powder is then extrusioncoated or dip coated with a solution or dispersion to provide acontinuous thermoplastic outer sheath, allowing free intermingling ofthe powder particles and the fibers within it.

The process of the present invention provides composite materials whichremain flexible and can be bent, and even knotted, without breaking.These flexible composite materials are thus suitable for braiding,weaving, and even knitting operations, opening up vast possibilities foruse of this material, possibilities beyond the reach of prior artmaterials because of the brittleness upon bending of the latter, asnoted above.

This invention is further described by reference to the accompanyingdrawings, wherein:

FIG. 1 shows an elevation view, partly in section, of apparatus usedaccording to the invention;

FIG. 2 is a sectional view of a coating die used in practicing theinvention;

FIG. 3 is a view of an article manufactured according to the invention,emplaced on a burst strength test apparatus; and

FIG. 4 illustrates, in cross section, a flexible composite manufacturedaccording to the invention.

Flexible composite materials can be obtained according to this inventionwith a fiber content of up to 50% by volume and 70% by weight.

The reinforcing fiber can be glass fiber, preferably of a size between160 and 320 tex, for covering with ordinary polymers as disclosedherein. Fibers of a size between 1200 and 2400 tex having been rised forcoating with extremely high fluidity polymers, also disclosed herein.

Other fibers used belong to the aramide family, such as superpolyamideswith an aromatic base, like Kevlar fibers. A preferred size is, forexample, 1580 decitex. Coating is carried out with the same types ofpolymers as for glass fibers.

Further, carbon fibers, the size of which is expressed in filaments (orin actual fact in kilofilaments), can also be used in this invention.Sizes in the 3 to 12 kilofilament range are best suited to production offlexible composite materials in accordance with this invention.

It will be apparent from the present disclosure that a wide variety ofthermoplastic powders, such as powdered polypropylene, polyethylene, PVC(polyvinyl chloride), polyester homo- and copolymers, polyvinylidenefluoride, and the like can be used in the present invention.

Among the resins in fine powder form introduced into the core of theroving of continuous fibers, those desired in certain embodiments of theinvention are polyamide powders, that is to say, polymers obtained bypolycondensation of 1 lactams (caprolactam, lauryl lactam, and the like)or from aminoacids such as the product of adipic acid and hexamethylenediamine, amino undecanoic acid, polyamidic copolymers, or the like.

The process of coating the continuous rovings loaded with fine powder,which is a characteristic of the invention, can be carried out withoutbringing the particles of fine powder, with which the roving isimpregnated, to their melting point. To accomplish this, either resinswith a melting point lower than that of the powder, such as 6/6.6/12terpolymer which melts at 105° C., or resins of the same type as thepowder but with a lower melt viscosity are used to meet the requirementsof the extrusion coating technique. Such resins also include polyamides6, 11 and 12, polypropylene, polyethylenes, PVDF, and polyesters,polysulfone, polyphenylene sulfide, plyimide, polyetherimide,polyetheretherketone.

In one version of the process according to the invention, the spread-outroving is impregnated with fine powder by passage through a bath ofaqueous or organic fine powder dispersion, as described in French patentapplication No. 81 03528, followed by drying. In another version, theroving containing the powder particles can be coated using one or theother of the following techniques, within the scope of the invention:Either passage through a melting unit containing the molten sheathingmaterial, or passage through a first bath of liquid monomer or oligomerand then through a second bath also containing liquid monomer oroligomer which reacts with that of the first bath to provide athermoplastic material. The pairs of monomers and/or oligomers used incertain preferred embodiments include polyols-polyisocyanates, givingpolyurethanes; polyurethanespolyureas; epoxy resins with hardener; epoxyresinsdicarboxylic polymers, or the like.

A further aspect of this invention resides in apparatus and processeswhich permit commercial realization of the objects of the invention,that is, to obtain flexible composite materials containing rovings ofcontinuous fibers impregnated to the core with thermoplastic fine powderwithout melting the particles of fine powder.

A process for production of a flexible composite material according tothe present invention is illustrated in FIG. 1.

Continuous roving of fibers 11 is unwound from feed reel 1 undertraction created by guide rolls 2 and 3. Via air-lock 7, the rovingenters fully sealed fluidization chamber 4, where finely powderedthermoplastic 5 is maintained in a state of fluidization by compressedair fed through conduit 6 into the lower chamber. The roving is spreadout or opened by known means such as rollers or splined cylinders, notshown in FIG. 1, but shown in French patent application No. 83 10632.The roving 11 becomes electrostatically charged by friction againstthese members and then progresses, now covered with fine powder, to thefluidization tank outlet, second air-lock 8.

Lock 8 is mounted to both fluidization chamber 4 and coating cross head9 by means of two thermal seals 16 and 17, shown in FIG. 2. These sealsallow maximum reduction of heat exchange between lock 8 and both thecoating cross head 9, a source of heat due to its positive heating (notshown), and the end of fluidization tank 4. This heat insulation avoidsgelling of the powder in contact with hot surfaces on which it mightmelt. In a preferred version of the apparatus, insulating seal 16 ismachined from a "Teflon" PTFE sheet and insulating seal 17 is sinteredzirconium oxide.

The tightness of the connections between fluidization tank 4, lock 8 andcross head 9 is intended to prevent particles of powder from escapingoutside the installation and causing difficulties owing to theirignition.

To supplement the back-flow by which any particle of powder not adheredto the roving by the electrostatic charge is returned to the tank, alight air current is generated in air-lock 8 by low volume pump 15. Thispump is of the roller type as supplied to laboratories by the W.Bachofen organization of Basel. The air pressure provided by this pumpis in the 0.10 to 0.30 bar range, and the flow can be reduced to aminimum, so as to generate a so-called "bubbling" air stream, byreducing the speed of rotation of the roller rotor with which it isequipped and which creates the air displacement in a flexible hosecyclically closed by the roller rotor.

Small cyclone separators, not shown, allow the excess air introduced bymeans of the pump 15 to escape with recovery of the small quantities ofpowder carried along by the air.

At the outlet of lock 8 the roving filled with the fine powder entersdie 9. It is desirable in certain preferred embodiments to provide wireguider 19 with an enlarged inlet 18 in order to avoid gelling of thepowder due to melting upon prolonged contact with metal markers of thecross head 9' and 9".

It is preferred to provide an inlet with a section between 1.5 and 3times that usually used for a wire guider when such a guide would beused to coat electric and telephone cables having a cross sectionequivalent to that of the roving.

Cross head 9' and 9" is fed with plastic sheathing material by extruder10 represented in the drawing by its barrel/die connection. The plasticsheathing material leaves the die in the form of so-called "sleeving",that is, the outlet diameter of the plastic sheath is determined by amandrel with a diameter greater than the external diameter of theroving. The sheath of molten material then shrinks due to traction onthe section leaving the die and comes into contact with the roving at adistance of about 5 mm from the outlet.

This coating head is advantageously positioned with a vertical axis. Thesection is almost cold after having travelled about 1 m through the air.It remains flexible and runs over slotted pulley 12 which is about 600mm in diameter. The impregnated roving is then guided by rollers 13 and13' and is wound onto take-up reel 14.

The composite 30 so formed comprises flexible sheath 31 covering roving33 of fibers impregnated with thermoplastic powder 32, as shown in FIG.4.

The following Examples are given to illustrate embodiments of theinvention as it is presently preferred to practice it. It will beunderstood that these Examples are illustrative, and the invention isnot to be considered as restricted thereto except as indicated in theappended claims.

EXAMPLE I

A 320 tex roving loaded with Atochem Orgasol 1002 fine powder isextrusion coated with a sheath of Atochem Rilsan AMNO resin.

The product characteristics are as follows:

Fine powder:

Polyamide 6

Bulk density: 0.50

Melt index: between 20 and 60 (1)

Sheath:

Polyamide 12

Density: 1.5

Melt index: between 10 and 40 (2)

The glass fiber used in this Example has a density of 2.40. A Maillefer30 extruder is used with a coating die of the type used for "sleeving"technique. The diameter of the die is 3.5 mm; the diameter of themandrel is 2.9 mm; the diameter of the wire guider, 1.5 mm; the inletdiameter, 2.4 mm. The die temperature is maintained at 195° C. with awinding speed of 100 m/min.

The flexible composite so produced has the following characteristics:

    ______________________________________                                                    Glass fibers                                                                          Fine powder  Sheath                                       ______________________________________                                        Percentage by weight                                                                        75        11           14                                       Percentage by volume                                                                        47        33           20                                       ______________________________________                                    

EXAMPLE II

A composite material in accordance with the invention, as shown in FIG.4, is used to reinforce plastic injection-molded components intended foruse under high internal pressure, e.g., hydraulic cylinders.

Such parts consist, for example, of a closed bottom cylinder with alength of 100 mm, a bore of 16 mm, and a wall thickness of 4 mm.

To allow comparative testing, these are produced using each one of thefollowing four techniques:

1. Part injection molded in polyamide 11 blended with short glass fibers(Atochem Rilsan ZMO).

2. Part produced using the same mold and plastic as in 1 but afterpositioning on the core of the mold prior to injection, single layer 23in FIG. 3 of flexible composite material, as in Example I, the compositebeing wound loosely around the core in contiguous spirals.

3. Part produced as in 2 but with a tension of 2 daN applied on thecomposite during winding.

4. Part produced as in 3 but with two superimposed thicknesses ofcomposite, the winding tension being maintained at 2 daN.

Referring to FIG. 3, to measure the burst pressures for the partsobtained using the four techniques described above, a hole is drilledthrough cylinder 21 and it is connected via fitting 22 to a source offluid under pressure. Piston 24 fitted with lipped seal 25 moves freelywithin the cylinder bore. For the purposes of the burst tests, piston 24is made from steel.

The apparatus consisting of the plastic cylinder and the steel piston isplaced in a rigid framework formed by two plates 26 and 26' and twouprights 27 and 27'. Dynamometer 28 is placed between the piston rod andplate 26'.

The burst pressure values can be computed from the measured forces,using the equation:

F=pS, where p is the pressure and S is the cross-sectional area. Becauseof the bore measurement chosen, the section S is 2.10⁻⁴ m², and thusF=2p.

Under these conditions, the average burst pressure values recorded areas follows:

    ______________________________________                                        Test type                                                                              Bursting force (daN)                                                                         Burst pressure (GPa)                                  ______________________________________                                        1          400          2                                                     2          450          2.25                                                  3        1,000          5                                                     4        1,100          5.50                                                  ______________________________________                                    

These data clearly demonstrate the improvement in mechanical strengthdue to the flexible composite material, especially when it is woundunder tension. Moreover, given the small winding diameter, it is evidentthat only a flexible composite according to the invention is suitablefor this operation.

What is claimed is:
 1. A composite article comprising a flexible sheathcovering a roving of fibers impreganted with thermoplastic powder, themelting point of the powder being the same or greater than that of theflexible sheathing material.
 2. A composite according to claim 1 whereinthe flexible sheath material selected is a thermoplastic resin.
 3. Acomposite according to claim 1 wherein the fiber is glass fiber, polymerfiber, or carbon fiber.
 4. A composite according to claim 1 wherein thepowder is polyamide, polypropylene, polyethylene, polyvinyl chloride,thermoplastic polyesters polyvinylidene fluoride, polyphenylene sulfide,polyetheretherketone, polyimide, polyetherimide, polysulfone.
 5. Aprocess for preparing a composite article according to claim 1, whichprocess comprises introducing the thermoplastic powder into and onto theperipheral surface of a continuous fiber roving so as to coat theindividual fibers of the roving and thereafter covering the roving witha sheath of flexible material, the melting point of which material isthe same or less than that of the thermoplastic powder.
 6. A processaccording to claim 5 wherein the covering with the sheath is carried outat a temperature below the melting point of the thermoplastic powder. 7.A process according to claim 5 wherein the flexible sheath is applied tothe roving by extrusion.